For products for high-frequency signal transmission, for example for coaxial cables, cables for LAN and printed wiring boards, the dielectric loss of the insulating material is an important factor.
The dielectric loss is a function of the permittivity (∈) and dielectric loss tangent (tan δ). The smaller that loss is, the more preferable the insulating material is.
Polytetrafluoroethylene [PTFE] is known as a material for insulators low in permittivity and dielectric loss tangent. PTFE has so far been molded by heating at above the melting point for effecting sintering.
With the recent advances in high-frequency signal transmission technology, high levels of transmission characteristics have become needed. For lowering the permittivity (∈) and dielectric loss tangent (tan δ) to thereby reduce the dielectric loss, it has been proposed that PTFE be used in a non-sintered or semi-sintered condition. However, non-sintered or semi-sintered PTFE had a problem in that it is poor in mechanical strength.
High-frequency transmission cables such as coaxial cables are sometimes subjected to terminal processing comprising cutting off the end portion of the coating in the lengthwise direction, with core wires being left as they are. The use of non-sintered or semi-sintered PTFE as an insulating coating material for coating core wires produces a poor terminal processability problem; namely, upon terminal processing, fiberization occurs at the cut end.
For maintaining the permittivity and dielectric loss tangent of the insulating coating material of coaxial cables at levels as low as possible and improving the mechanical strength thereof, it has been proposed that the sintering degree in an insulating coating material composed of non-sintered PTFE and sintered PTFE be inclined in the radial direction so that the sintering degree on the outer surface side of PTFE may be increased (cf. e.g. Patent Document 1: Japanese Kokai Publication H11-31422).
For improving the terminal processability of insulating coating materials, it has been proposed that the terminal portion alone, which is to be subjected to terminal processing, be made of fully sintered PTFE and the other portion be made of non-sintered PTFE or semi-sintered PTFE so as to give a gradient in sintering degree in the lengthwise direction of the core wires (cf. e.g. Patent Document 2: Japanese Kokai Publication H11-283448).
These insulating coating materials are lower in dielectric loss tangent than those made of sintered PTFE alone as the resin but still have a problem: they have a portion constituted of non-sintered PTFE and, therefore, insufficient in mechanical strength and/or terminal processability.
It has also been proposed that a mixture of PTFE (a) with a number average molecular weight of 10±5×105 and PTFE (b) with a number average molecular weight of 45±10×105 is sintered at a temperature not lower than the melting point of PTFE (a) but lower than the melting point of PTFE (b) to give an insulating coating material comprising semi-sintered PTFE (cf. e.g. Patent Document 3: Japanese Kokai Publication 2001-357729). This insulating coating material is low in permittivity and dielectric loss tangent and improved in terminal processability. However, higher levels of mechanical strength and terminal processability are now required.
In this way, various investigations have been made concerning the sintering degree, among others, of PTFE as an insulating coating material for high-frequency signal transmission products. As for other fluororesins than PTFE and other resins, however, they are higher in both permittivity and dielectric loss tangent than PTFE, so that they have not yet been made targets of investigation as materials of insulators.